Referring to FIG. 1, in an ESA printing machines, ink is transferred from an engraved printing cylinder 10 to an underside of a web 11 of non-conducting material aided by electrostatic attraction. A roller 12 positioned above the web 11 provides the electrostatic attraction of the ink to the web 11. In some applications, this roller 12 is referred to as an impression roller because it forms a nip 14 with the printing cylinder 10 and contacts the web 11. The impression roller usually has a resilient outer covering of synthetic rubber of 60 to 95 Shore A durometer hardness that bears against the web 11 and the printing cylinder 10. The engraved printing cylinder 10 may rotate through a reservoir 16 holding ink or coating material, the thickness of which is controlled by doctor blade 17.
In other applications, the ESA roller is spaced from the web in a machine known as a gap coating machine, also known as a meniscus or bead coating machine. In this machine the web is in contact with the impression roller but not the engraved cylinder. The engraved cylinder meters the coating to the nip region where there is a very small, but precise gap or air space. Such machines can use a chrome-plated copper, a laser engraved ceramic cylinder or a smooth chrome-plated cylinder with a doctor blade that controls the coating material on the cylinder.
Voltage is applied to the ESA roller either through a slip ring arrangement or through a third roller in the machine known as the voltage applicator roller 18 as seen in FIG. 1. ESA rollers normally have at least two layers, an insulating base material 19 on the metal core 20 to prevent leakage to ground, and a semiconductive synthetic rubber material 21. If the roller core can be insulated from ground, only a semiconductive rubber layer is needed 21. A typical ESA printing machine, including an impression roller, is described and shown in Adamson, U.S. Pat. No. 3,477,369 and Hyllberg et al., U.S. Pat. No. 4,493,256, issued Jan. 15, 1985.
Technical problems in ESA rollers are mechanical wear, chemical deterioration, and heating at higher speeds of press operation. Heating can cause greater aging of the rubber layer typically used as the outer layer in impression rollers. When ESA rollers with rubber covering become worn, their diameter changes, which may affect printing operations. Eventually, wear on the outer covering will require replacement or recovering of the roller.
Recently, in Hyllberg, U.S. patent application, Ser. No. 07/973,447, filed Nov. 9, 1992, which is referred to above, there was disclosure of ceramic materials for a charging roller for a copying machine. A difference there is that the charge donor roller more often has a voltage applied directly to its core and does not need to be grounded. In comparison, an ESA roller is preferably insulated from the rest of the machine and has a voltage applied from another roller.
An advantage of ceramic for ESA applications is that ceramic may be formed in thinner layers than semiconductive rubber. A ceramic layer maintains its operating dimensions over substantial use. The present ESA application is a coating operation in which a coating must be applied in a uniform thin layer of liquid coating on a web.
A further problem in the art is vibration of the ESA roller at higher machine speeds. In Carlson, U.S. Pat. No. 5,256,459, issued Oct. 26, 1993, a fiber-reinforced composite tube is disclosed for roller applications. Such a core exhibits good dampening characteristics at higher speeds of rotation. Such a tube, however, has not been known to be combined in a roller with ceramic layers, due to difficulties in bonding the two materials.
The present invention is intended to overcome the limitations of the prior art by providing methods and constructions of ceramic ESA rollers for both coating and printing applications.